SPECiES And SPECiATion 223
Gametic isolation occurs when gametes of different species fail to unite. This
barrier is important in many externally fertilizing species of marine invertebrates
that release eggs and sperm into the water. Because cell surface proteins determine
whether or not sperm can adhere to and penetrate an egg, divergence in these
proteins can result in gametic isolation [69]. Among species of abalones (large gas-
tropods), the failure of heterospecific eggs and sperm to unite is related to the high
rate of divergence in the amino acid sequences of both lysin (the sperm protein
that dissolves the egg’s vitelline envelope) and the vitelline envelope protein with
which it interacts (see Chapter 10) [32]. In cases that fall in between premating and
postmating isolation, mating occurs but fertilization does not. In many groups of
insects and some other taxa, the genitalia of related species differ in morphology.
There is evidence that females terminate mating, and prevent transfer of sperm,
if a male’s genitalia do not provide suitable tactile stimulation (FIGURE 9.10) [22].Postzygotic barriers
Postzygotic barriers consist of reduced survival or reproductive rates of hybrid
zygotes that would otherwise backcross to the parent populations. These barriers can
be classified as either extrinsic or intrinsic, depending on whether or not their effect
depends on the environment. Intrinsic isolation is based on interactions between
genes from two populations, and is often more permanent than extrinsic isolation.
Extrinsic postzygotic isolation is often based on reduced survival because of
ecological factors. In some cases, the parent species are adapted to different envi-
ronments; the hybrid may be poorly adapted to both. A simple example is provided
by hybrids between species of Heliconius butterflies that are distasteful to birds and
have different patterns of warning coloration. Birds learn to associate common
color patterns with distastefulness, but are likely to attack butterflies with rare,
unfamiliar phenotypes, such as hybrids. Researchers placed artificial butterflies,
with wing patterns of two species and their F 1 hybrid, in a tropical forest, and
scored the number that were damaged by attacking birds [60]. Those with hybrid
color patterns were more frequently attacked (FIGURE 9.11).FIGURE 9.10 Differences in genitalia
can contribute to reproductive isolation
between species if copulation between
them occurs. (A) The genital arch in male
Drosophila is involved in transferring
sperm to females. Its shape differs among
closely related species, as the close-ups
show: (B) D. sechellia, (C) D. mauritiana,
(D) D. simulans. This morphological fea-
ture is almost the only one by which these
species differ. (A from [53]; B, C, and D
courtesy of J. R. True.)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_09.11.ai Date 11-19-2016Proportion attacked0.060.080.020.040
melpomeneF 1 hybrid cydnoFIGURE 9.11 Model butterflies with the
color pattern of the F 1 hybrid between
Heliconius melpomene and H. cydno
were attacked by birds significantly more
frequently than those with the pattern of
either parent. The low survival of hybrids is
an example of postzygotic isolation caused
by an extrinsic factor. (From [60].)
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_0910.ai Date 11-02-2016(A) (B)(C) (D)100 μm09_EVOL4E_CH09.indd 223 3/23/17 9:36 AM